The Global Positioning System (GPS) is part of a satellite-based navigation system developed by the United States Defense Department. A fully operational GPS includes up to 24 satellites in non-geosynchronous orbit, each transmitting spread spectrum, L-band carrier signals on both the L1 (1575.42 MHz) and L2 (1227.6 MHz) carrier frequencies. The Galileo System is a satellite-based navigation system initiated by the European Union. The Open Service (OS) is one of a number of combined services provided by Galileo. Open Galileo is defined for mass-market applications and offers up to three separate signal frequencies.
Precise code (P-code) is a portion of the GPS signal that is typically utilized by the United States military. P-code has a 10.23 MHz bit rate and a period of precisely one week. P-code is encrypted (encrypted P-code is referred to as Y-code) and reset every seven days to prevent unauthorized use. Clear/Acquisition code (C/A-code) is a portion of the GPS signal located in the center of the GPS band that is available for civilian and commercial use. C/A-code has a 1.023 MHz bit rate and a period of one millisecond. Each GPS satellite broadcasts unique C/A-code to distinguish itself from all other GPS satellites. The open Galileo signal structure is also located in the center of the band with a Binary Offset Carrier (BOC) BOC(1, 1) signal structure. Military code (M-code) is a portion of the GPS signal that is modernized and designed to protect utilization of GPS by the United States military and its allies. The M-code signal structure was designed with the intent to jam the center of the GPS band (known as prevent or blue force jamming) to deny enemy C/A-code receivers the ability to track. While the M-code signal was designed with inherent jamming protection, the C/A-code, P(Y)-code, and open Galileo signals were not.
One technique for dealing with blue force jamming, which is typically up to about 2-3 MHz wide, is to include a notch filter in the signal processing chain. However, this may prevent C/A-code from being tracked by friendly forces and cause severe distortion to the P(Y)-code correlation function. Anti-Jam (AJ) processing including spatial nulling, space-time adaptive processing (STAP), and space-frequency adaptive processing (SFAP) may be incorporated. However, the degrees of freedom of the anti-jam system may be reduced by blue force jamming, especially when multiple blue force jammers are utilized in a given area. When the degrees of freedom are used up, the STAP and SFAP techniques are forced to notch the signal at the center of the band, having the same negative effect on the C/A-code and P(Y)-code signals. Moreover, spatial nulling is ineffective under these conditions.
Thus, it would be desirable to jam the C/A-code and open Galileo signals to prevent enemy use while still allowing friendly receivers to track these signals and minimizing distortion to P(Y)-code signals.